The field of nuclear medicine has long been concerned with techniques of diagnosis wherein radiopharmaceuticals are introduced into a patient and the resultant distribution or concentration thereof, as evidenced by gamma ray intensities, is observed or tracked by an appropriate system of detection. An important advantage of the diagnostic procedure is that it permits non-invasive investigation of a variety of conditions of medical interest. Approaches to this investigative technique have evolved from early pioneer procedures wherein a hand-held radiation counter was utilized to map body contained areas of radioactivity to more current systems for simultaneously imaging substantially an entire, in vivo, gamma ray source distribution. In initially introduced practical systems, scanning methods were provided for generating images, such techniques generally utilizing a scintillation-type gamma ray detector equipped with a focusing collimator which moved continuously in selected coordinate directions, as in a series of parallel sweeps, to scan regions of interest. A drawback to the scanning technique resides in the necessarily longer exposure times required for the derivation of an image. For instance, such time elements involved in image development generally are overly lengthy to carry out dynamic studies of organ function.
By comparison to the rectilinear scanner described above, the later developed "gamma camera" is a stationary arrangement wherein an entire region of interest is imaged at once. As initially introduced the stationary camera systems generally utilized a larger diameter sodium Iodide, Na I (TI) crystal as a detector in combination with a matrix of photomultiplier tubes. A multiple channel collimator is interposed intermediate the source containing subject of investigation and this scintillation detector crystal. When a gamma ray emanating from the region of investigative interest interacts with the crystal, a scintillation is produced at the point of gamma ray absorption and appropriate ones of the photomultiplier tubes of the matrix respond to the thus generated light to develop output signals. The original position of gamma ray emanation is determined by position responsive networks associated with the outputs of the matrix. For additional information concerning such cameras, see:
I. Anger, H.O., "A New Instrument For Mapping Gamma Ray Emitters," Biology and Medicine Quarterly Report UCRL-3653, 1957.
A continually sought goal in the performance of gamma cameras is that of achieving a high resolution quality in any resultant image. Further, it is desirable to achieve this resolution in combination with concomitant utilization of a highly versatile radionuclide or radiolabel, 99m-Technetium, having a gamma ray or photon energy in the region of 140 keV. A broadened clinical utility for the cameras also may be realized through the use and image identification of radiopharmaceuticals exhibiting more than one photon energy level. With such an arraignment, two or a plurality of diagnostic aspects simultaneously may be availed the operator. For example, in carrying out myocardial imaging, the above-identified 99m-Technetium might be utilized in conjunction with 111-Indium, the latter contributing photon energy in the regions of 173 and 247 KeV. Similarly, 81-Rubidium, exhibiting photon energy in the range of 350 KeV might be utilized in conjunction with 81-Krypton, the latter having gamma ray energy at about 120 KeV. The noted dual energy characteristic of 111-Indium also might be utilized to achieve two aspects of diagnostic data.
The resolution capabilities of gamma cameras incorporating scintillation detector crystals, inter alia, is limited both by the light coupling intermediate the detector and phototube matrix or array as well as by scatter phenomena of the gamma radiation witnessed emanating from within the in vivo region of investigation. Concerning the latter scattering phenomena, a degradation of resolution occurs from scattered photons which are recorded in the image of interest. Such photons may derive from Compton scattering into trajectories wherein they are caused to pass through the camera collimator and interact photoelectrically with the crystal detector at positions other than their point of in vivo derivation. Should such photon energy loss to the Compton interaction be less than the energy resolution of the system, it will effect an off-axis recordation in the image of the system as a photopeak photon representing false spatial information or noise. As such scattered photons record photopeak events, the noise increases and consequent resolution quality of the camera diminishes. For the noted desirable 140 KeV photons, the scintillation detector type camera energy resolution is approximately 15 KeV. With this resolution, photons which scatter through an angle from 0.degree. to about 70.degree. will be seen by the system as such photopeak events.
A continuing interest in improving the resolution qualities of gamma cameras has lead to somewhat extensive investigation into imaging systems incorporating relatively large area semiconductor detectors. Such interest has been generated principally in view of theoretical indications of an order of magnitude improvement in statistically limited resolution to provide significant improvements in image quality. In this regard, for example, reference may be made to the following publications: II. R. N. Beck, L. T. Zimmer, D. B. Charleston, P. B. Hoffer, and N. Lembares, "The Theoretical Advantages of Eliminating Scatter in Imaging Systems," Semiconductor Detectors in Nuclear Medicine, (P. B. Hoffer, R. N. Beck, and A. Gottschalk, editors), Society of Nuclear Medicine, New York, 1971, pp. 92-113.
III. R. N. Beck, M. W. Schuh, T. D. Cohen, and N. Lembares, "Effects of Scattered Radiation on Scintillation Detector Response," Medical Radioisotope Scintigraphy, IAEA, Vienna, 1969, Vol. 1, pp. 595-616.
IV. A. B. Brill, J. A. Patton, and R. J. Baglan, "An Experimental Comparison of Scintillation and Semiconductor Detectors for Isotope Imaging and Counting", IEEE Trans. Nuc. Sci., Vol. NS-19, No. 3, pp. 179-190, 1972.
V. M. M. Dresser, G. F. Knoll, "Results of Scattering in Radioisotope Imaging" IEEE Trans. Nuc. Sci., Vol. NS-20, No. 1, pp. 266-270, 1973.
Particular interest on the part of investigators has been paid to detectors provided as hybridized diode structures formed basically of germanium. To derive discrete regions for spatial resolution of impinging radiation, the opposed parallel surfaces of the detector diodes may be grooved or similarly configured to define transversely disposed rows and columns, thereby providing identifiable discrete regions of radiation response. Concerning such approaches to treating the detectors, mention may be made of the following publications:
VI. J. Detko, "Semiconductor Dioxide Matrix for Isotope Localization", Phys. Med. Biol., Vol. 14, No. 2, pp. 245-253, 1969.
VII. J. F. Detko, "A Prototype, Ultra Pure Germanium Orthogonal Strip Gamma Camera," Proceedings of the IAEA Symposium on Radioisotope Scintigraphy, IAEA/SM-164/135. Monte Carlo, October 1972.
VIII. R. P. Parker, E. M. Gunnerson, J. L. Wankling, and R. Ellis, "A Semiconductor Gamma Camera with Quantitative Output," Medical Radioisotope Scintigraphy.
IX. V. R. McCready, R. P. Parker, E. M. Gunnerson, R. Ellis, E. Moss, W. G. Gore, and J. Bell, "Clinical Tests on a Prototype Semiconductor Gamma-Camera," British Journal of Radiology, Vol. 44, 58-62, 1971.
X. Parker, R. P., E. M. Gunnerson, J. S. Wankling, R. Ellis, "A Semiconductor Gamma Camera with Quantitative Output," Medical Radioisotope Scintigraphy, Vol. 1, Vienna, IAEA, 1969, p. 71.
XI. Detko, J. F., "A Prototype, Ultra-Pure Germanium, orthogonal-Strip Gamma Camera," Medical Radioisotope Scintigraphy, Vol. 1, Vienna, IAEA, 1973, p. 241.
XII. Schlosser, P. A., D. W. Miller, M. S. Gerber, R. F. Redmond, J. W. Harpster, W. J. Collis, W. W. Hunter, Jr., "A Practical Gamma Ray Camera System Using High Purity Germanium," presented at the 1973 IEEE Nuclear Science Symposium, San Francisco, November 1973; also published in IEEE Trans. Nucl. Sci., Vol. NS-21, No. 1 February 1974, p. 658.
XIII. Owen, R. B., M. L. Awcock, "One and Two Dimensional Position Sensing Semiconductor Detectors," IEEE Trans. Nucl. Sci., Vol. NS-15, June 1968, p. 290.
In the more recent past, investigators have shown particular interest in forming orthogonal strip matrix detectors from p-i-n semiconductors fashioned from an ultra pure germanium material. In this regard, reference is made to U.S. Pat. No. 3,761,711 as well as to the following publications:
XIV. J. F. Detko, "A Prototype, Ultra Pure Germanium, Orthogonal Strip Gamma Camera," Proceedings of the IAEA Symposium on Radioisotope Scintigraphy, IAEA/SM-164/135, Monte Carlo, October, 1972.
XV. Schlosser, P. A., D. W. Miller, M. S. Gerber, R. F. Redmond, J. W. Harpster, W. J. Collis, W. W. Hunter, Jr., "A Practical Gamma Ray Camera System Using High Purity Germanium," presented at the 1973 IEEE Nuclear Science Symposium, San Francisco, November 1973; also published in IEEE Trans. Nucl. Sci., Vol. NS-21, No. 1, February 1974, p. 658.
High purity germanium detectors promise numerous advantages both in gamma camera resolution as well as practicality. For instance, by utilizing high purity germanium as a detector, lithium drifting arrangements and the like for reducing impurity concentrations are avoided and the detector need only be cooled to requisite low temperatures during its clinical operation. Readout from the orthogonal strip germanium detectors is described as being carried out utilizing a number of techniques, for instance, each strip of the detector may be connected to a preamplifier-amplifier channel and thence directed to an appropriate logic function and visual readout. In another arrangement, a delay line readout system is suggested with the intent of reducing the number of preamplifiers-amplifier channels, and a technique of particular interest utilizes a charge splitting method. With this method or technique, position sensitivity is obtained by connecting each contact strip of the detector to a charge dividing resistor network. Each end of each network is connected to a virtual earth, charge sensitive preamplifier. When a gamma ray interacts with the detector, the charge released enters the string of resistors and divides in relation to the amount of resistance between its entry point in the string and the preamplifiers. Utilizing fewer preamplifiers, the cost and complexity of such systems is advantageously reduced. A more detailed description of this readout arrangement is provided in:
XVI. Gerber, M. S., Miller, D. W., Gillespie, B., and Chemistruck, R. S., "Instrumentation For a High Purity Germanium Position Sensing Gamma Ray Detector," IEEE Trans. on Nucl. Sci., Vol. NS-22 No. 1, February, 1975, p. 416
To achieve requisite performance and camera image resolution, it is necessary that substantially all sources of noise or false information within the system be accounted for. In the absence of adequate noise resolution, the performance of the imaging systems may be compromised to the point of impracticality. Until the more recent past, charge-splitting germanium detector arrangements have not been considered to be useful in gamma camera applications in consequence of thermal noise anticipated in the above-noted resistor divider networks, see publication VII, supra. However, as will be evidenced in the description to follow, such considerations now are moot.
Another aspect in the optimization of resolution of the images of gamma cameras resides in the necessarily inverse relationship between resolution and sensitivity. A variety of investigations have been conducted concerning this aspect of camera design, it being opined that photon noise limitations, i.e. statistical fluctuations in the image, set a lower limit to spatial resolution. Further, it has been pointed out that the decrease in sensitivity witnessed in conventional high resolution collimators may cancel out any improvements sought to be gained in image resolution. A more detailed discourse concerning these aspects of design are provided, for instance, in the following publications:
XVII. E. L. Keller and J. W. Coltman, "Modulation Transfer and Scintillation Limitations in Gamma Ray Imaging," J. Nucl. Med. 9, 10, 537-545 (1968)
XVIII. B. Westerman, R. R. Sharma, and J. F. Fowler, "Relative Importance of Resolution and Sensitivity in Tumor Detection," J. Nucl. Med. 9, 12 638-640 (1968)
Generally, the treatment of the signals derived at the entrance detection portion of gamma cameras involves a form of spatial or coordinate identification of photons reaching the detector and additionally, a form of analysis of the energy of radiation reaching the detector. Spatial analysis may be carried out by difference summing circuits, while energy determination is carried out by additive summing circuits. Further, pulse height analyzers may be utilized as one discriminating component of a system for determining the presence of true or false imaging information. In any of the systems both treating noise phenomena and seeking a high integrity of spatial information, a control is required which carries out appropriate noise filtering while segregating true from false information. In addition to the foregoing, it is necessary that the "through-put rate" of the system be maximized in order that it may accommodate a highest number of bits or pulses representing spatial and energy data.
Another operational phenomenon tending to derogate from the spatial resolution quality performance of the cameras is referred to as "aliasing". This phenomenon represents a natural outgrowth of the geometry of the earlier-noted orthogonal strip germanium detector. A more detailed discussion of this aspect of the gamma cameras is provided at:
XIX. J. W. Steidley, et al., "The Spatial Frequency Response of Orthogonal Strip Detectors," IEEE Trans. Nucl. Sci., February, 1976.
To remain practical, it is necessary that the imaging geometry of stationary type gamma cameras provide for as large a field of view as practical. More particularly, such considerations require a camera field of view large enough to encompass the entire or a significant extent of the profiles of various organs of interest. Because of limitations encountered in the manufacture of detector crystals, for instance, high purity germanium crystals, the size of solid state detector components necessarily is limited. As a consequence, composite detector configurations are required which conjoin a plurality of smaller detector components to provide an imaging field of view or radiation acceptance geometry of effectively larger size. However, such union of a multitude of detector components must be carried out without the concurrent generation of noise phenomena and without a significant loss of image information validity and acuity. For instance, in the latter regard, spatial information must have a consistency of meaning across the entire extent of an ultimately displayed image of an organ, otherwise, clinical evaluation of such images may be encumbered. Preferred arrangements for inter-coupling the discrete detector components within an overall array thereof is described in a copending application for United States Patent by M. S. Gerber and D. W. Miller, entitled "Gamma Camera System With Composite Solid State Detector" filed Apr. 27, 1976, Ser. No. 680,754 and assigned in common herewith.
The control systems utilized with gamma cameras having multi-component detectors further are called upon to collect image data therefrom at an optimum rate while evaluating the validity thereof and assigning it an appropriate address function. Such address assignment may vary in nature depending upon the selected mode of circuit interrelationship of the discrete detector components with the array. An additional function of the control system is to identify the spatial position of the detector-photon interaction for select but different energy levels. This requires a technique for normalizing the spatial labels of such signals while properly evaluating the energy level states thereof as representing valid image information. The rapidity with which this data is treated, as by assigning spatial regional factors to it, as well as evaluating it for validity becomes a particularly important aspect of the control systems where they are contemplated for use in clinical dynamic function studies. With such studies, dynamic alterations in an image component occuring within any segment of the image area should be followed closely in correspondence with the actual movement of the image source. Accordingly, efficient image signal treatment by the camera system is required.